Due to global spread of information and telecommunication devices such as smartphones and progress in the Internet of Things (IoT), demand for high-capacity optical transmission technology is increasing. On the transmitter side, high-capacity transmission is being achieved by multi-level optical modulation and wavelength division multiplexing (WDM) schemes. On the receiver side, coherent optical transmission designed for coherent detection of optical signals associated with digital signal processing is a key technology for high-speed high-capacity optical communication.
In general, an optical transmitter (Tx) has an optical modulator and a driver for driving the optical modulator, and an integrated coherent receiver (ICR) is provided as an optical receiver (Rx). The modulator package size becomes large as the number of modulation levels or the number of multiplexed wavelengths increases. The same applies to the receiving-end ICR package.
With a configuration of either as separate chips or packaged driver, modulator, and ICR, mounted on a print circuit board, the size of the optical transceiver module would increase. The optical transceiver module size can be reduced by integrating the optical modulator and the ICR on the same photonic integrated circuit (IC), and then incorporating the photonic IC and a driver formed as an electric or electronic circuit into the same package.
In optical transmission using multiple channels for improving the transmission rate, radio frequency (RF) signals input to the optical transmitter are regulated such that the output timings of optical signals to be generated are in alignment with each other between the channels. The optical signals received at the optical receiver are also regulated such that the output timings of the optical-to-electrical converted RF signals are in alignment with each other between the channels. With this configuration, a skew between the channels is minimized on both the transmitter side and the receiver side.
One known technique is compensating for a skew between optical signal channels by adjusting the length of the electrical wirings. See, for example, patent documents 1 to 3.